1. Field of the Invention
Ihe present invention relates to an auxiliary circuit breaker. More particularly, it relates to an auxiliary circuit breaker mechanically interlocked with the switching operation of a main circuit of electric devices such as low or high voltage electromagnetic contactors or various type of circuit breakers.
2. Description of the Prior Art
Ihe conventional auxiliary circuit breaker will be described with reference to FIGS. 1 to 6.
In FIG. 1, a base (1) formed by plastic molding includes six chambers (2) which are insulated from each other. Each chamber contains a switching element, partition barriers (3), ('3) for insulating each chamber, a window (4) and fitting holes (5), (5'). FIG. 2 illustrates in detail base (1) in which a first stopper (7) and a second stopper (8) are formed so as to limit the movement of a cross bar (6) which is located at the center of the base (1) and is movable along the longitudinal axis of the base. The base also includes a stationary contact (20), (20') in each chamber (2). The cross bar, formed by plastic molding, has six rectangular holes (10) with which a movable contact engages, an elongated rectangular hole (11) at the lower portion, a seat (12) for a spring (17), which seat (12) is formed at the bottom of the hole (11), a first projection (13) which is brought into contact with the first stopper (7), and a second projection (14) which is brought into contact with the second stopper (8) of the base (1). A known spring (15) for the contact and a spring washer (16) are held in the rectangular hole (10). The movable contact is placed at a lower position in case of a normally closed contact system (which occupy three positions from the bottom of the base shown in FIG. 1) and is placed at an upper position in the case of normally open contact system (three positions from the top of the base shown in FIG. 1). The releasing spring (17) and a cover holder (18) acting as a seat for the spring are provided at the rectangular hole (11) formed at the lower portion of the cross bar (6). The base (1) is covered by a dust-proof cover (19) made of a transparent plastic material.
FIGS. 1 and 2 show stationary contact (20), (20') schematically. FIG. 3 shows the stationary contact (20) in detail in which it is formed by bending a metal sheet to provide three portions. A contact point (21) is bonded on the central portion and a threaded hole (22) is formed in one side of the stationary contact. The stationary contact is fitted into a seat (9) of the base (1) by utilizing its own spring action and screws (23), (23') serving as output terminals (FIG. 1) are screwed into the threaded hole (22), thus the movement of the stationary contact in the transverse direction in FIG. 1 is prevented.
In the structure described above, the cross bar (6) is normally pressed down by the releasing spring (17) to the stop position which makes the top of the elongated rectangular hole (11) of the cross bar (6) contact with the cover holder (18). The conventional auxiliary circuit breaker is adapted to cooperate mechanically with the switching operation of a main circuit breaker (not shown). The main circuit breaker is mainly of a DC magnet-saving resistor type meeting the demands of reduction of shock on switching, long life and minaturization of an electromagnet.
FIG. 5 illustrates a circuit diagram of the main circuit breaker wherein the symbol MC designates the coil of the main circuit breaker, (MC-b) designates the normally closed contact of an auxiliary circuit breaker which has a greater overtravel so that the operation of the contact is delayed from another normally closed contact during an opening operation, (R) designates a saving resistor which is connected in parallel to the normally closed contact (MC-b) of the auxiliary circuit breaker and (SW) designates a command switch placed outside.
The operation of the main circuit breaker is carried out as follows: When the command switch placed outside is turned-on, a current flows to (SW)-(MC-b)-(MC) to apply the full voltage to the operating coil (MC). A movable iron core (not shown) is attracted to close the main contact of the main circuit breaker, then the normally closed contact (MC-b) is opened to insert the saving resistor thereby passing a current through (SW)-(R)-(MC). As a result, a movable contact is attracted to the stationary contact to render the main contact of the main circuit breaker in closing condition. As well known, attractive force is great in the condition that the movable contact is attracted to the stationary contact whereby a sufficiently great attractive force is maintained even though voltage is shared by the saving resistor (R) and the operating coil (MC), respectively.
When the command switch (SW) is turned off to prevent the feeding of current, the movable contact detaches from the stationary contact, thus causing disconnection of the main contact of the main circuit breaker whereby the normally closed contact (MC-b) of the auxiliary contact returns to the original closed state because of the zero voltage.
The conventional auxiliary circuit breaker has the following disadvantage. When the normally closed contact (MC-b) is actuated to turn off the coil current of the main circuit breaker so as to apply shared voltage to the resistor (R) and the coil (MC) respectively, the attractive force of the movable iron core suddenly reduces because the normally closed contact (MC-b) is opened before the closing operation of the main contact of the main circuit breaker. The reduction in attractive force may cause unreliable contact operation of the main contact or a suspending state of the main contact at the crossing point (C) due to the chattering of the command switch (SW) or voltage drop because the attractive force is lower than repulsive force in same portion as it is clear from a repulsive-attractive force characteristic shown in FIG. 6. Thus, faults such as melting or adhesion of the main contact have occurred.
A cross bar (2) having a longer rectangular hole (10) has been proposed and practically used to delay the detachment of the normally closed contact (MC-b). The proposed circuit breaker has failed to eliminate completely the unreliable contact operation or the suspending state at crossing point (C) of the main contact because a gap for the contact is too small to prevent the interruption of the coil current.